TLRs play a pivotal role in acquired immunity by triggering the maturation of DC to competent antigen presenting cells (APC), capable of priming naive T cells. Because TLR3 triggers a unique activation pathway, we hypothesized that antigen-bearing DC activated by dsRNA would generate an acquired response different from DC activated with other PAMPs. We have therefore embarked upon a project to determine how DC respond to different length dsRNA oligos, and how these DC activate naive T cells, especially in comparison with the TLR9 PAMP, CpG DNA. To date we have found that the activation of mouse DC depends upon dsRNA length, and that DC respond differently to dsRNA than to CpG DNA with regard to cytokines produced, surface phenotype, and activation of naive T cells. Studies are underway to determine the intracellular compartments where dsRNA meets TLR3 in different DC subsets, and to compare the in vivo priming capacities of immunogens containing antigen and different length dsRNA oligos, or CpG DNA. An overall goal of these studies is to fine-tune PAMP adjuvants to generate immune responses specific for particular classes of pathogens. In the case of dsRNA, a viral PAMP, we would expect the response to be especially effective against viral pathogens. Based on our previous observation that DC also express histamine receptors, we hypothesized that histamine would have an effect on the maturation process. In testing this hypothesis we found that histamine profoundly alters the cytokines released by DC during TLR induced maturation, and as a result, histamine greatly diminishes the capacity of DC to produce Th1 cells from naive T cells. Mast cells are the major source of histamine, and they are often located in close proximity to DC. We therefore hypothesized that mast cell degranulation at a site of immunization would alter the nature of the immune response by acting on neighboring DC. By using a mouse model with adoptively transferred transgenic T cells, we have demonstrated that procedures that induce mast cell degranulation affect T cell polarization in vivo, and by using mice that are deficient in mast cells, we have now shown that mast cells are responsible for this effect. We have shown that mast cell degranulation at the site of immunization affects the isotype of the antibodies produced, and that DC from such mice lose their capacity to generate Th1 cells.